FlightGear flight simulator is the open-source,freeware simulator developed by the FlightGear Project. The first release was in 1997,and so far there was a lot of releases.

Despite being freeware,FGFS is a very good and realistic simulator. Along with the simulator it self,everything else related to it is also free to download,use and develop further. To download or if you want to learn more about it,you can do it at the official FlightGear web site,
www.flightgear.org
.

The main place to download various aircraft,more than 400 of them,is here:
github.com/FGMEMBERS
.
Good thing about it is that you don't have to register to download the aircraft you want.

Scenery can be downloaded in two ways,on the web site or by using the TerraSync system within the simulator. Also,depending on your PC's power,you can choose between default and HD scenery.

FlightGear also has it's own multiplayer network,but unfortunately no active virtual airlines,and no support for large mp networks like VATSIM or IVAO.